Line 739 and line 748 of threads_import.php would need to be changed. Say you wanted to do 50 topics at a time. You'd need to change line 739 to 50 instead of 20, and then line 748 you'd change to 51 instead of 21.
As for only importing the new topics created after the database dump. Unforunately not. All of the importers are just capable of doing a full import.
